Symbol Timing Estimation for Coherent Polarization Multiplex Optical Receivers

1. A method for retrieving symbol timing information with an apparatus in a POLMUX system, comprising the steps of polarisation splitting a received polmux signal with a polarization splitter into two received polmux signal components (x, y; Xt, Yt) with orthogonal polarizations; rotating the orthogonal polmux signal components (x, y; Xt, Yt) by fixed rotation parameters (Rot 0 , Rot 1 , Rot 2 ) with at least three polarization rotators to obtain at least three pairs of rotated signal components (X 0 , Y 0 ; X 1 , Y 1 ; X 2 , Y 2 ) with different polarizations; squaring the obtained rotated signal components (X 0 , Y 0 ; X 1 , Y 1 ; X 2 , Y 2 ) with squarer units and determining a rotated signal component (X 1 ) having the best phase information quality; selecting with selectors both corresponding rotated squared signal components (X 1 , Y 1 ) of the determined signal component (X 1 ) for further processing; deriving phase information (Phi_x, Phi_y) of both selected signal components (X 1 , Y 1 ) with at least one phase information extractor; and combining the derived phase information (Phi_x, Phi_y) to achieve the symbol timing information for correcting the phase or calculating an optimized timed signal, wherein the polarization splitter, polarization rotators, squarer units, selectors and at least one phase information extractor are part of the apparatus.

2. The method according to claim 1 , wherein the received orthogonal signal components (x, y) are sampled and converted into digital samples (Xt, Yt) of both polarities to be further processed.

3. The method according to claim 1 , wherein the received polmux signal components (x, y) and/or sampled polmux signal components (Xt, Yt) are rotated by rotators with fixed parameters.

4. The method according to claim 3 , wherein the polmux signal is rotated by three parameters Rot ⁢ ⁢ 0 : { α = 0 δ = 0 ⁢ ⁢ Rot ⁢ ⁢ 1 : { α = π ⁢ / ⁢ 4 δ = 0 ⁢ ⁢ Rot ⁢ ⁢ 2 : { α = π ⁢ / ⁢ 4 δ = π ⁢ / ⁢ 2 or corresponding parameters with α: polarization mixing angle, δ: polarization ellipticity angle.

5. The method according to claim 1 , wherein the rotated signal components (X 0 , X 1 , X 2 ; Y 0 , Y 1 , Y 2 ) are squared and DFT processed for deriving phase information (Phi_x, Phi_y).

6. The method according to claim 5 , wherein only the rotated signal components (X 0 , X 1 , X 2 ) associated to only one polarity are processed for selecting rotated signal components (X 1 , Y 1 ) to be processed for determining the phase information (Phi_x, Phi_y).

7. The method according to claim 6 , wherein complex coefficients (SXI 0 , SXQ 0 ; SXI 1 , SXQ 1 ; SXI 2 , SXQ 2 ) retrieved by DFT processing of the rotated and squared signal components (X 0 , X 1 , X 2 ) are once more squared and gained amplitudes are valuated to determine and to select the rotated signal components (X 1 , Y 1 ) or the retrieved complex coefficients (SXI 0 , SXQ 0 ; SXI 1 , SXQ 1 ; SXI 2 , SXQ 2 ) with the best phase information quality.

8. The method according to claim 1 , wherein the polmux signal components (XI, XQ; YI, YQ) are highpass filtered and/or chromatic dispersion compensated.

9. The method according to claim 1 , wherein the rotated signal components (X 0 , Y 0 ; X 1 , Y 1 ; X 2 , Y 2 ) are parallel processed.

10. An apparatus for retrieving symbol timing information in a polmux system including a polarization splitter for splitting a received polmux signal into two received polmux signal components (x, y; Xt, Yt) with orthogonal polarizations, means for sampling and converting a received optical polmux signal into digital samples representing polmux signal components (X t , Y t ), the apparatus further comprising at least three polarization rotators receiving said polmux signal components (X t Y t ,) and outputting at least three rotated complex signal components (X 0 , Y 0 ; X 1 , Y 1 ; X 2 , Y 2 ); squarer units receiving said rotated signal components (X 0 , Y 0 ; X 1 , Y 1 ; X 2 , Y 2 ) and outputting amplitudes (AX 0 , AX 1 , AX 2 , AY 0 , AY 1 , AY 2 ) of said rotated signal components; wherein said apparatus is further configured for selecting both corresponding rotated squared signal components (X 1 , Y 1 ) of the determined signal component (X 1 ) for further processing; deriving phase information (Phi_x, Phi_y) of both selected signal components (X 1 , Y 1 ); and combining the derived phase information (Phi_x, Phi_y) to achieve the symbol timing information for correcting the phase or calculating an optimized timed signal.

11. The apparatus according to claim 10 , further comprising DFT units processing said amplitudes and retrieving complex coefficients (SXI 0 , SXQ 0 ; SXI 1 , SXQ 1 ; SXI 2 , SXQ 2 ; SYI 0 , SYQ 0 ; SYI 1 , SXY 1 ; SYI 2 , SYQ 2 ); squarers receiving said complex coefficients (SXI 0 , SXQ 0 ; SXI 1 , SXQ 1 ; SXI 2 , SXQ 2 ) of at least one polarity and deriving their amplitudes; a maximum detector receiving said amplitudes and determining the complex coefficients (SXI 1 , SXQ 1 SYI 1 , SYQ 1 ) with the best phase information quality; selector circuits selecting said complex coefficients (SXI 1 , SXQ 1 SYI 1 , SYQ 1 ); phase information extractors deriving phase information of the selected complex coefficients (SXI 1 , SXQ 1 SYI 1 , SYQ 1 ); and a phase information adder calculating the phase information (Phi) for timing processing.

12. The apparatus according to claim 10 , further comprising pre-filters receiving polmux signal components (XI, XQ; YI, YQ) and outputting high-pass filtered polmux signal components fed to interpolators or rotators.

13. The apparatus according to claim 11 , further comprising interpolators arranged downstream in series of the pre-filters calculating intermediate samples.

14. The method according to claim 5 , wherein only complex coefficients (SXI 1 , SXQ 1 ) retrieved by DFT processing of said rotated Signal components (X 1 , Y 1 ) associated to only one polarity are processed for selecting the complex coefficients to be processed for determining the phase information (Phi_x, Phi_y).